U.S. Pat. No. 5,251,053 discloses a radio communication system, particularly a cellular mobile-radio system, which comprises two optical transmitters having different frequencies. By forming the difference of the two frequencies, a carrier frequency is provided. A frequency regulator is provided for maintaining the frequency difference. Disadvantages are the use of two optical devices and the relatively costly frequency regulator.
From "Electronics Letters", 27th Feb. 1992, Vol. 28, No. 5, page 461, "Simultaneous Oscillation of Wavelength-Tunable, Singlemode Lasers Using an Er-Doped Fiber Amplifier", it is known to use a fiber ring laser with a bypass for generating a plurality of frequencies. Microwave generation, particularly with regard to the formation of a frequency difference, is not dealt with in that reference.